1. Field of the Invention
Embodiments disclosed herein relate generally to oil-based wellbore fluids and methods of using such fluids. In particular, embodiments disclosed herein relate generally to oil-based fluids being emulsifier-free.
2. Background Art
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
In most rotary drilling procedures the drilling fluid takes the form of a “mud,” i.e., a liquid having solids suspended therein. The solids function to impart desired rheological properties to the drilling fluid and also to increase the density thereof in order to provide a suitable hydrostatic pressure at the bottom of the well. The drilling mud may be either a water-based or an oil-based mud.
The selection of the type of drilling fluid to be used in a drilling application involves a careful balance of both the good and bad characteristics of the drilling fluids in the particular application and the type of well to be drilled. The primary benefits of selecting an oil-based drilling fluid, also known as an oil-based mud, include: hole stability, especially in shale formations, formation of a thinner filter cake than the filter cake achieved with a water-based mud, excellent lubrication of the drilling string and downhole tools, and penetration of salt beds without sloughing or enlargement of the hole, as well as other benefits that should be known to one of skill in the art. Despite the many benefits of using oil-based muds, they have some disadvantages, including high initial and operational costs as well as environmental concerns.
Oil-based muds typically contain some water, either from the formulation of the drilling fluid itself, or water may be intentionally added to affect the properties of the drilling fluid or mud. In such water-in-oil type emulsions, also known as invert emulsions, an emulsifier is used to stabilize the emulsion. In general, the invert emulsion may contain both water soluble and oil soluble emulsifying agents. Typical examples of such emulsifiers include polyvalent metal soaps, fatty acids and fatty acid soaps, and other similar suitable compounds that should be known to one of ordinary skill in the art.
Emulsifiers and oil wetting agents are added to conventional oil-based muds to emulsify the water phase in the oil phase and to ensure that all of the solids in the mud are wetted by the oil. Specifically, weighting agents, an essential component of oil-based muds, must remain oil wet. The necessity for such weighting agents (typically barite) to be oil-wet is because when water-wet, this material agglomerates and rapidly sediments out of the fluid. When such sedimentation occurs, it can be difficult to maintain a constant mud weight, the consequence of which could be a severe well control problem.
However, the majority of formations drilled are naturally water-wet but when contacted by the emulsifiers present in oil-based muds are easily changed to oil-wet. Such alteration in the wettability of the formation can reduce the permeability of the formation to oil and, in addition, increase oil retention on cuttings, leading to increased disposal considerations. Disposal of oil-wet cuttings is particularly a concern when cuttings include water-wet shale, which are frequently drilled with an oil-based fluid to avoid complications such as bit balling when drilling with a water-based fluid. When drilling with an oil-based fluid through shale, oil-wetting surfactants can cause penetration of oil into the pores of the shale (particularly along places of weakness), causing dispersion of the cuttings into fine particles. The surfactant-induced dispersion leads to a build up of undesirable “low gravity solids” and high viscosity, the only remedy for which is to dilute the mud with additional quantities of oil (increasing waste volume and costs associated with cleaning the fluids for disposal).
The ability to eliminate such emulsifiers would allow cuttings and reservoir formations drilled using oil-based muds to remain in their natural, water-wet state. This would lead to both lower oil retention on cuttings and to smaller reductions in permeability. Thus, while the presence of emulsifiers and other oil wetting agents have been considered to be essential components of conventional oil-based muds, the presence of such materials can alter the wettability of reservoir formations thereby reducing their permeability to oil and increasing oil retention. Accordingly, there exists a continuing need for oil-based wellbore fluids that are emulsifier-free.